Elusive isotopic signatures of recharge and assimilation in the arc-adjacent silicic volcanic systems of the Loicas Trough, Argentina

Abstract

Open system processes including recharge and assimilation are fundamental drivers of volcanic activity and can provide the trigger for major eruptions. We explore these open system processes in silicic volcanic centres of the Loicas Trough, an arc-adjacent volcanic province, situated between the Andean main and back arc in the Southern Volcanic Zone Argentina. We model the effect of open system processes on bulk radiogenic Pb, Nd, Hf, Sr isotopes and in situ major element and ⁸⁷Sr/⁸⁶Sr compositions of plagioclase crystals, using Magma Chamber Simulator (MCS). Our results and MCS models reveal the amount of assimilation and crustal heterogeneity overwhelm potential source variations, and assimilation trends are largely controlled by residual wallrock mineralogy. Plagioclase phenocrysts indicate several disequilibrium events, including recharge as seen from zonation, resorption and sieve textures and An% profiles. In situ Sr isotope measurements displayed limited intra-crystal variation. MCS models of bulk compositions and plagioclase zonation reveal that recharge events are generally elusive in bulk rock and mineral isotopic data. The compositional impact of recharge is smaller than the natural variation of the data trends, despite clear textural evidence for recharge events. This has important implications for using geochemical proxies to identify open system magma chamber processes and eruption trigger mechanisms in silicic volcanic centres.